In a large number of medical procedures, it is important to monitor hemodynamic parameters in a patient. For example, in procedures such as hemodialysis, a filter is used to remove selected particles and liquids from the bloodstream of a patient, but if the particles and liquids are removed too quickly, the vascular system of the patient may collapse.
Thus, the treatment of patients by hemodialysis is frequently accompanied by acute symptoms or complications such as hypotension, severe muscular cramps and lightheadedness. Although the pathophysiology of intradialytic morbid events (IME) is complex and multifactoral, hypovolemia has been suggested to play a triggering role.
Intravascular volume depletion results from an imbalance between the rates of extra corporeal ultrafiltration and refilling the blood compartment. As red cell volume remains essentially constant during hemodialysis, changes in hematocrit (R. R. Steuer et al., Hematocrit as an Indicator of Blood Volume and a Predictor of Intradialytic Morbid Events, ASAIO J., 1994, 38: M181-M185); hemoglobin (M de Vries et al., Continuos Measurement of Blood Volume During Hemodialysis by an Optical Method, ASAIO J., 1992, 38: M181-M185); and ultrasound velocity (U.S. Pat. No. 5,230,341 to Polashegg) have been traditionally viewed as inversely related to changes in the circulating blood volume. This relationship has been used in attempts to project dialysis induced hypotension.
Existing technology for monitoring total blood volume in a patient during hemodialysis relies upon the assumption that changes in volume are inversely proportional to an arterial concentration of large blood particles, such as hematocrit, hemoglobin, or total proteins that cannot defuse through the dialysis membranes. However, it is known that arterial hematocrit alone does not represent whole blood volume concentration. In the capillaries, arterioles and venule hematocrit may be less than half the arterial concentration. (Gibson J G, et al. The Distribution Of Red Cell And Plasma In Large And Minute Vessels Of The Normal Dog, Determined By Radioactive Isotopes Of Iron And Iodine, J. Clin. Invest. 25:848, 1946). Therefore, observed changes in hematocrit (hemoglobin) may not relate to the actual volume changes but to the redistribution of red cells in the vascular space. Thus, the present technologies may not measure total blood volume changes accurately.
In the Abstract by Jacobson S H, et al., (Double Indicator Dilution Estimation of Extravascular Lung Water and Cardiac Output During Hemodialysis, Journal of American Society of Nephrology, Vol. 4, No. 3, Abstract, p.357, 1993), the blood volume that is located in the right heart, lungs and left heart was measured by injecting dye into a venous port and recording dilution curves during the blood withdraw through an arterial port into a dencitometer. This approach fails to accommodate the significant amount of time that the indicator travels in extracorporeal tubing. Thus, this measurement may not provide a sufficiently reliable measurement for diagnosis of IME. These reliability issues also exist when the measurements are performed with intravenous injections in a venous catheter and the dilution curve is recorded on an extracorporeal tubing system that withdraws blood from an artery.
Therefore, the need exists for a method and apparatus for assisting the prediction of an onset of IME during procedures such as hemodialysis. The need also exists for the identification and monitoring of a physiological parameter which can assist in the prediction of complications during hemodialysis, intensive care unit treatments and surgical procedures.